1. Field of the Invention
The present invention relates to an automatic gain control (AGC) system for use with a receiver in digital cellular radio applications.
2. Description of the Prior Art
Known AGC systems use a feedback configuration to control their gain. If this is applied to a direct conversion receiver however, the AGC voltage has to be derived from the inphase (I) and quadrature (Q) phase components of the input signal. For stable operation the gain of the AGC loop should fall to unity well below the lowest frequency to be amplified, for example 1 KHz. The loop will therefore take many milliseconds to stabilise. Any attempt to speed up the response will result in distortion or instability.
Increasing the speed of an AGC system above that which is possible using feedback can only be achieved by the use of feedforward techniques. In such an arrangement, the input signal is split between a gain controlled amplifier and an envelope detector. The output of the envelope detector is suitably scaled and level shifted, and is used to control the amplifier gain such that the output is constant. As this is an open loop system there is no stability or distortion problems, and the response time is limited only by the time taken for the detector to reach the peak of the input waveform.
The main problem with feedforward techniques is that the gain control and detector characteristics must accurately track each other over the AGC range required. This is very difficult to achieve over more than about 20 dB in an analogue implementation, whereas the required range for digital cellular radio applications is typically nearer 80 dB. For this reason, feedforward techniques are seldom used in practice.
Accordingly, an aim of the present invention is to provide an AGC system which enables a fast settling time to be achieved and which overcomes the above mentioned problems in an efficient and simple manner.